1. Field of the Invention
This invention pertains to methods and apparatus for determining background content of an image of a scanned document. More particularly, this invention relates to a method for determining the document background and suppressing its effects on document copies. This invention also relates to methods for adjusting the dynamic range of the image.
2. Description of Related Art
In copier systems, copying a document (or more generally, an original) while suppressing the substrate of the original is often required, such as when the original is printed on colored paper. Background detection can be performed on just the leading edge of the document or the whole document. However, whole page background detection generally requires pre-scanning the entire original. The detected background can be removed by adjusting the gain of the scanned image and clipping the values that exceeds the system processing range.
One approach to estimating the original's background value is by performing a running average in an area of the original where there is just background. This approach also blocks using any video below a selectable threshold just in case non-background material is included in the selected area. This running average approach works well with pure background, but performs poorly when non-background material is present, especially when the non-background material occurs at the edge of the window where the final average is most affected.
Automatic background suppression senses the background and automatically suppresses the background before final printing. Conventional automatic background suppression systems generate a histogram of the document using standard methods and then calculate the mean and standard deviation. This often involves significant amounts of calculation to determine the gain needed to eliminate the background noise.
To further prevent gross loss of image detail of an input document, the dynamic range should also be adjusted. FIG. 11A shows an image of a newspaper photograph with no background noise elimination or dynamic range adjustment.